Several common disorders of the brain, spinal cord, and peripheral nervous system arise due to abnormal electrical activity in biological (neural) circuits. In general terms, these conditions may be classified into:                (1) Conditions such as epilepsy, in which electrical activity is dysregulated, and recurrent activity persists in an uncontrolled fashion;        (2) Conditions such as stroke or traumatic injury, in which an electrical pathway is disrupted, disconnecting a component of a functional neural circuit; and        (3) Conditions such as Parkinson's disease, in which neurons in a discrete region cease to function, leading to functional impairment in the neural circuits to which they belong.        
When the electrical lesion is focal and relatively discrete, as is very often the case, effective diagnosis and treatment of such conditions depends on precise localization of the lesion and, when possible, restoration of normal electrophysiologic function to the affected region.
A variety of well-established techniques exist for localizing electrical lesions in the brain, each of which has specific limitations:                (1) Imaging techniques such as magnetic resonance imaging (MRI) and computed tomography (CT) constitute entirely noninvasive methods of examining brain tissue, and many functional lesions (including strokes, anatomic abnormalities capable of causing seizures, and foci of neuronal degeneration) can be detected and precisely localized using such imaging modalities. Not all functional lesions can be detected these using imaging modalities, however, as these techniques do not image electrical activity. Furthermore, these imaging techniques lack temporal resolution, and provide no mechanism for therapeutic electrophysiologic intervention.        (2) Electromagnetic recording techniques such as electroencephalography (EEG) and magnetoencephalography (MEG) are entirely noninvasive techniques that provide excellent temporal resolution of electrical activity in the brain. For this reason, EEG is currently the gold standard modality for detection of seizure activity. The spatial resolution of such techniques is limited, however, both due to physical distance of electrodes from the brain, and by the dielectric properties of scalp and skull. The spatial resolution of EEG is better for superficial regions, and worse for neural activity deep within the brain.        (3) Electrocorticography (ECoG), or intracranial EEG, is a form of electroencephalography that provides improved spatial resolution by placing recording electrodes directly on the cortical surface of the brain (in conventional EEG, by contrast, electrodes are positioned on the scalp). This modality is frequently used during neurosurgical procedures, to map normal brain function and locate abnormal electrical activity, but it requires craniotomy, temporary surgical removal of a significant portion of the skull, in order to expose the brain surfaces of interest, and exposes patients to the attendant risks of brain surgery. Furthermore, while electrical activity near the cortical surface of the brain can be mapped with reasonable spatial resolution, electrical activity deep within the brain remains difficult to localize using ECoG.        (4) “Depth electrodes” record electrical activity with high spatial and temporal precision, but such electrodes record only from small volumes of tissue (small populations of neurons), and their placement requires disruption of normal brain tissue along the trajectory of the electrode, resulting in irreversible damage or destruction of some neurons. As such electrodes are placed surgically, in a hypothesis-driven manner, the number of such electrodes that can be safely placed simultaneously is limited.        (5) Deep brain stimulation (DBS) electrodes, the stimulating analog of recording depth electrodes, electrically stimulate brain regions with millimetric precision. They are implanted using minimally invasive surgical techniques, and can be effective in conditions such as Parkinson's disease, in which neuronal dysfunction is confined to a small, discrete, and unambiguous region of the brain.        
While the foregoing list is not exhaustive, it provides a general overview of the range of techniques presently available for electrical recording and stimulation of the living human brain.
In practice, all neural recording and stimulation techniques involve design trade-offs among a number of primary factors:                (1) Spatial resolution;        (2) Temporal resolution;        (3) Degree of invasiveness; and        (4) Optimization for electrical recording or electrical stimulation.        
Blindness and visual impairment are often caused by disorders in the eyes, leaving the brain optic radiations and visual cortex intact but disconnected from normal input. Consequently, in forms of blindness due to macular degeneration, glaucoma, and diabetic retinopathy, and several other conditions, though the retina or optic nerve may be diseased, electrical stimulation of the optic radiations and visual cortex can still generate predictable visual sensations.